[FIX] mail: correctly update new and removed subtypes when updating subscription

Currently, the subtype checked selection is not saved when the user adds and
removes subtypes. Indeed either an addition is performed, either a removal
but having both is not correctly taken into account when updating an existing
subscription.

How to reproduce
  * click on the pencil icon to edit the subscription of the follower;
  * as an example default "Discussions" will be the only one selected;
  * check "Notes" and save;
  * -> this works;
  * click again on the pencil icon and uncheck "Notes" and check "Activities"
    then save;
  * -> this does not work as only removal is performed (Activities is not
    checked);

The problem only occurs when you unchecked subtypes and checked other ones.

This commit fixes that behavior by taking into consideration both new
subtypes and subtypes to remove.

LINKS

Task ID-2205643
Task ID-2241688
COM PR #56775

Co-Authored-By: Thibault Delavallee <tde@odoo.com>
Co-Authored-By: Shahista Pathan <sat@odoo.com>